The present invention relates to a method for manufacturing a magnetic disk to be mounted on a magnetic disk device such as a hard disk drive (HDD) or the like, or a method for manufacturing a glass substrate for a magnetic disk.
A magnetic disk is known as one of recording media for information processing equipment and is widely used for a hard disk drive (HDD). Such a magnetic disk has a substrate and a thin film such as a magnetic layer and the like on a substrate. As the substrate, an aluminum substrate has been employed. However, recently, in order to respond to the demand for increased recording density, recent tendency is that glass substrates have been widely used instead of the aluminum substrates because a spacing between the magnetic head and the magnetic disk can be narrowed in the glass substrates as compared with aluminum substrates. Also, each surface of the glass substrates is precisely polished, which makes it possible to realize high recording density and to lower a flying height of the magnetic recording head as low as possible.
As described above, high smoothness of the magnetic disk surface is required and indispensable for lowering a flying height (flying amount) necessary for realizing high recording density. In order to obtain high smoothness of the magnetic disk surface, requirement is directed to high smoothness of the substrate surface. However, realizing high recording density on the magnetic disk has become difficult now only by simply polishing the substrate surface in high precision. That is to say, no matter how much the substrate surface is polished, high smoothness cannot be obtained when a foreign object is adhered on the substrate. Of course, removal of foreign objects has been performed conventionally, but the technology has reached a state wherein foreign objects conventionally neglected or allowed on the substrate are recognized as a problem in today's high-density requirements.
The glass substrate having high smoothness can be obtained by fine polishing by the use of a cerium oxide polishing agent. However, after the polishing process using a cerium oxide polishing agent, foreign objects which are protruded often remain without being removed with a normal cleaning process. This results in a problem wherein the surface roughness thereof cannot be reduced. The protruded foreign objects are frequently formed by polishing abrasives remaining on the substrate.
As for a technique for removing the protruded foreign objects formed by adhered cerium oxide abrasives, with the technology field of glass substrates for a information recording medium, for example, performing sulfate cleaning after polishing with cerium oxide abrasives has been proposed (see Japanese Unexamined Patent Application Publication (JP-A) No. 2000-348338). Further, for example, with the technology field such as information recording media, liquid crystal or organic EL displays, photo-masks, and the like, a technique for cleaning the glass substrate with cleaning chemical solution including three components of acid, reducer, and fluorine ion, has been proposed (see Japanese Unexamined Patent Application Publication (JP-A) No. 2004-059419).
Recently, with hard disk drives (HDD), information recording density of 60 gigabits/inch2 is demanded. This relates to a phenomenon wherein, in addition to needs for conventional computer storage drives, hard disk drives have been mounted in cellular phones, navigation systems, digital cameras, and so forth.
In such new applications, the chassis space for mounting a hard disk drive is markedly small as compared with computers, so hard disk drives need to be reduced in size. To this end, the diameter of a magnetic disk to be mounted on a hard disk drive needs to be reduced in size. For example, with computer applications, 3.5 inch or 2.5 inch magnetic disks have been able to be employed, but with the above new application, smaller diameter magnetic disks than those, such as 0.8 inch through 1.8 inch for example, are employed. Thus, even with a case of reducing magnetic disks in size, information capacity of a predetermined level or more should be increased, which accelerates augmentation of information recording density.
Also, in order to use a restricted disk area more effectively, hard disk drives employing an LUL (Load/Unload) method have been employed instead of hard disk drives employing the conventional CSS (Contact Start and Stop) method. With the LUL method, when a hard disk drive is stopped, the magnetic head thereof is evacuated to a tilting table called a ramp, positioned outside of the magnetic disk, and when the hard disk drive is activated, following the magnetic disk starting rotations, the magnetic head is slid on the magnetic disk from the ramp, and is subjected to flying flight to perform recording and reproduction. When the hard disk drive is stopped, the magnetic head is evacuated to the ramp outside of the magnetic disk, and then rotation of the magnetic disk is stopped. These series of movements are called LUL movements. With the magnetic disks for a hard disk drive employing the LUL method, a contact sliding region (CSS region) as to the magnetic head needs not to be provided unlike the CSS method, and a recording/reproducing region can be enlarged, which is preferable for realizing increase of information capacity.
In order to improve information recording density under such a situation, spacing losses need to be infinitely reduced by decreasing the flying amount of the magnetic head. In order to achieve information recording density of 60 gigabits/inch2 or more, the flying amount of the magnetic head needs to be reduced to 10 nm or less. With the LUL method unlike the CSS method, a convex/concave shape for the CSS method needs not to be provided on the magnetic disk surface, resulting in readily realizing smoothness on the magnetic disk surface. Accordingly, with the magnetic disk employing the LUL method, the flying amount of the recording head can be further decreased as compared with that employing the CSS method, and increase in S/N ratio of recording signals can be realized, which contributes to realization of high recording capacity of the magnetic disk device.
Due to further decrease of the flying amount of the magnetic head according to introduction of the recent LUL method, even with extremely low flying amount of 10 nm or less, stable movements of the magnetic disk have been demanded. However, upon the magnetic head being subjected to flying over the magnetic disk surface with such an extremely low flying amount, a problem has resulted wherein a fly-stiction frequently occurs. Fly-stiction is a malfunction wherein the flying attitude of the magnetic head suddenly becomes unstable during recording/reproducing, causing abnormal fluctuation in recording signals and reproducing signals. This fly-stiction especially readily occurs with the magnetic head which performs flying with an NPAB (negative pressure air bearing surface) slider, i.e., a negative pressure slider. The magnetic head including a negative pressure slider has an advantage wherein stable flying flight can be performed even at low flying amount of 10 nm or less, but this causes strong negative pressure upon the magnetic head undersurface (i.e., surface facing the magnetic disk). Accordingly, this makes for a situation wherein fly-stiction readily occurs.
Further, with the flying amount of the magnetic head reaching 10 nm or less, with the magnetic head on which a magneto-resistance effect-type reproducing device, e.g., a TMR (tunneling magneto-resistance) type reproducing device is mounted, a problem is created in that thermal asperity readily occurs. Upon thermal asperity occurring, the error rate drastically deteriorates, and it is hard to perform recording/reproducing of information with a predetermined recording density, e.g., 60 gigabits/inch2 or more, for example.
In light of these situations, the present inventor has attempted to develop a magnetic disk, and a glass substrate for a magnetic disk, which can perform safely recording/reproducing without crashing, fly-stiction, and thermal asperity, even in the event of the magnetic head flying within 10 nm. For example, the present inventor has attempted to remove adhered polishing agent in a precise manner, by refining polishing abrasives to perform accurate mirror polishing process, and then preparing a cleaning chemical solution in a subsequent cleaning process. For example, the present inventor has used various cleaning techniques including cleaning that utilizes the techniques of the above Japanese Unexamined Patent Application Publication (JP-A) No. 2000-348338, and Japanese Unexamined Patent Application Publication (JP-A) No. 2004-059419, but has found out that these problems cannot be always prevented in a sure manner.